Digital Control of Power S u p p l i e e Opportunities and Constraints Petri Vallittu
Teuvo Suntio
Seppo J. Ovaska
EFORE Oyj P. 0. Box 61 FIN-0221 1 ESPOO, FINLAND petri.vallittu @ efore.fi
EFORE Oyj Joukontie 42 FIN-01400 VANTAA, FINLAND teuvo.suntio@ efore.fi
Helsinki University of Technology Institute of Intelligent Power Electronics Otakaari 5 A FIN-02150 ESPOO, FINLAND
[email protected]
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Abstract In this paper, we present a study of the opportunities and constraints of digital control in power supplies. The advantages and disadvantages between analog and digital controllers in switched-mode power supplies are discussed in detail. The effects of a digital controller on device’s reliability and integration level are also discussed. A 48 V, 500 W rectifier having a switching frequency of 100 kHz was used as a practical case example. This switched-mode rectifier consists of two converter stages, i.e., AC/DC and DCDC converters. Dynamic characteristics for digital controllers in both converters were studied by extensive simulations. Based on the simulation results, the values of the required sampling frequencies were determined. According to the obtained results and careful analyses made in our study, it can be stated that digital control is certainly a viable alternative also in power supplies. However, before digital controllers will become dominant in high-volume power supply products, low cost microcontrollers or signal processors, which are tailored for the specific application, are needed to reduce the existing cost barrier.
I. INTRODUCTION
Control of switched-mode power supplies has traditionally been based on integrated pulse width modulator (PWM) circuits. Mostly, the development in the control of switched-mode power supplies has been a direct result of the development and availability of these control ICs.
11. SWITCHED-MODE POWER SUPPLY
Power supplies are increasingly equipped with microcontrollers and DSPs to implement digital control. It is well known that digital control has several advantages compared to analog control. The main advantages of a digital approach over its analog counterpart are; lower sensitivity to changes in the environment such as temperature, supply voltage fluctuation, aging of components, and so on, and the possibility of a lower part count, thus increasing the integration level and improving the reliability as well as reducing assembling costs. Shortened design cycles can also be achieved, A short design cycle is a very important
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It can be seen from international conference proceedings published in the last few years, that the interest in digital control of power supplies has clearly increased. Too often, however, new techniques or methods, in the field of electronics, are rejected by practicing engineers due to the lack of adequate knowledge. Our paper provides a possibility for a power supply designer to recognize the important opportunities and constraints related to digital control of power supplies. One of the aims of this paper is to remove possible prejudices toward digital control.
This paper is organized as follows. Section I1 describes the structure of the switched-mode rectifier, which was used in this study. In section 111, some design considerations on the digital control of the AC/DC and DC/DC converters are introduced. Section IV presents the applied simulation models and the obtained results for the ACDC converter. Simulation models and results for the D C D C converter are presented in section V. Advantages and disadvantages between analog and digital controllers in switched-mode power supplies are discussed in section VI. Section VI1 reviews briefly the situation in the DSP development. Finally, section VI11 concludes the paper.
Advances in very-large-scale integration (VLSI) have made possible the expanding use of digital computers in many real-time application areas. It is not only the steady price reduction that has made them attractive in various new application areas, but also the great functional development of digital signal processors (DSPs). One of the latest new DSP application areas is power electronics. Remarkable progress has been made, e.g., in advanced AC motor control. DSPs have become common components in modern motor control applications [ 1-21. Digital signal processing techniques are not only used to replace conventional analog signal processing and control functions but they open up totally new opportunities, such as fuzzy control.
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advantage, especially in companies which develop customer-specific products.
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Sampling rate plays a key role in digital control. In this study, the dynamic requirements of digital voltage controllers were studied by simulations in the MATLAB@ and SIMULINK” environment. To get a reliable verification for simulation results, a 48 V, 500 W telecom rectifier was used as a reference example, see Fig. 1 [3].
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MATLAB and SIMULINK are registered trademarks of The MathWorks Inc.
Fig. 1. Schematic structure of a 500 W telecom rectifier
In Fig. 1, the rectifier consists of two converter stages, i.e., the ACDC and DCDC converters. The main structure is very typical for off-line power supplies. In our rectifier, both converters use the switching frequency of 100 Wz. The two functions of the ACDC converter, i.e., the power factor corrector, are (1) to improve the quality of the input current along with high power factor and (2) to maintain the intermediate voltage within certain limits suitable for the subsequent conversion stage. The main target in the design of the power factor corrector (PFC) is to comply with agency regulations relating to harmonic input currents [4-51. A comprehensive overview of the PFC topologies can be found, e.g., in [6]. Fig. 2 presents a principle scheme of a typical boost type PFC, which was used in our rectifier. The control scheme in Fig. 2 utilizes average current mode control [7].
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Fig. 2. Principle scheme of a typical boost type PFC; KI and scaling factors.
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In the DCDC converter, this telecom rectifier uses a forward converter to convert the intermediate voltage (380 VDC) to the output DC voltage of 48 V. The forward converter stage uses peak current mode control, also called simply current mode control. Current mode control is the most typical control scheme in DCDC converters. Compared to conventional voltage mode control current mode control has several advantages [SI, such as inherent feedforward characteristic and automatic peak switch current limitation.
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111. DESIGN CONSIDERATIONS ON THE DIGITAL CONTROL OF ACDC AND DCDC CONVERTERS Dynamic requirements of the DCDC converter differ greatly from the requirements of the PFC converter, so both converters with their controllers were simulated and analyzed. The DCDC converter has a much higher voltage loop bandwidth than the ACDC converter. Due to the inherent feedforward feature of the current mode control in the subsequent conversion stage a small and slowly varying deviation in the intermediate voltage does not cause any significant perturbation in the output of the rectifier. Therefore, it can be stated that the main target in the design of the PFC is to comply with agency regulations relating to harmonic input currents [4-51.
A. AC/DC Converter (PFC) In high power factor preregulator circuits similar to the one presented in Fig. 2, the input current distortion consists mainly of the third harmonic [9], arising from two sources: (1) input current fails to track perfectly the sine wave reference signal, and (2) the current reference signal is distorted by the second harmonics from the output voltage feedback and from the feedforward voltage. In this study, we concentrated on analyzing the output voltage feedback. Due to the fluctuating input power and finite energy storage element, i.e., a capacitor, the intermediate voltage contains a low frequency ripple component at twice the line frequency. In conventional controllers, this ripple affects the input current waveform unless the open-loop cross-over frequency is kept well below the line frequency (typically 10-20 Hz). The dynamic response of an intermediate voltage is sacrificed due to this limitation of the voltage control loop bandwidth. Higher voltage error gain improves the dynamic response but increases distortion. Because of this poor dynamic response, recently, the research related to power factor correctors has mainly concentrated on improving the dynamic response (see, e.g., [lo]). By means of improved dynamic response the output capacitor value can be lowered only if the hold-up time requirements are still fulfilled. Lower capacitor value results in reduced cost and weight, but it also leads to increased ripple voltage.
Remarkable benefits from better dynamics compared to conventional voltage controllers are here questionable because the following converter stage is designed to handle these deviations in the intermediate voltage within specified limits. Therefore, we prefer that the design of a digital controller for PFC should begin with a concept of a robust and simple configuration which meets the primary task, i.e., low harmonic currents. In addition, the implementation of the PFC should naturally be cost effective and reliable.
B. DC/DC Converter (Forward Converter) The requirements in the output stage of the rectifier are far more strict than in the PFC. The specified response times are much shorter in the DC/DC converter than in the PFC converter. In addition, several special conditions such as overload-protection need fast reaction times and therefore, must be taken into consideration. However, an additional current limiting circuitry is still needed to set an absolute maximum for output current. One special condition can be seen in Fig. 3. The output of the rectifier is short-circuited and we can see how the output current behaves. Interesting, in this particular example, is that the analog controller is not capable of rapidly performing active current control, but the total settling time is about 18 ms. Therefore, the sampling rate requirements for the digital controller are not based on the absolute current limiting. This means that by means of an external current limiting comparator the required speed for the current controller is reduced, making it possible to use the average current mode control instead of peak current mode control.
Fig. 4. (a) Boost converter and (b) continuous mode behavior model.
The second converter stage in rectifiers represents a constant power load for the PFC converter, so constant power load was also used in the simulations. Fig. 5 presents the SIMULINK block diagram by means of which the dynamics of the voltage controller was analyzed.
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Fig. 5.Control scheme of the PFC in SIMULINK environment.
The current controller in Fig. 5 is a simple PI (Proportional Integral) controller. The output signal of the current controller is the duty cycle D, which is allowed to vary between zero and one, Our low-frequency boost model gets this signal as its input parameter. The amplitude of the line voltage was kept constant so the feedforward signal was not used and the line-regulation was not examined. The applied PFC converter parameters are presented in Table I. P
TABLE I PFC CONVERTER PARAMETERS
Fig. 3. An example of the measured output current waveform when the output of the rectifier is short-circuited.
IV.SIMULATION MODELS AND RESULTS FOR BOOST TYPE PFC (AC/DC) To examine the performance of a voltage controller, a low-frequency model for boost topology was constructed. Fig. 4 presents a continuous mode model for a boost converter. Resistors rL, rd, r,s, and r, are the series resistances of the inductor L, diode d, switch SW and capacitor C, respectively.
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R
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As it was mentioned above, the dynamic requirements of the voltage controller in PFC are low, so a reasonably low sampling frequency is adequate. Fig. 6 presents the analog error amplifier stage which controls the intermediate voltage. The parameters of the analog voltage controller are: iJref= 3 V, Ri= 660 kL2,Rd = 5.25w2, Rf = 470 WZ, C, = 47 nF. In the simulations, the step response was tested. Load was changed from 10 % to 100 % of the nominal 550 W power, and vice versa.
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Also some other control schemes, such as simple PI controller, Sample-and-hold, and Notch filter, were studied [ 111. Sample-and-hold is simple and allows low computational burden due to low sampling frequency, i.e., twice the line frequency. However, sampling must be synchronized to line-frequency zero-crossings, which requires some additional functions.
Fig. 6. Voltage error amplifier: (Ur@=3 V, Ri = 660 kQ, Rd = 5.25 kQ, R, = 470 kQ, CO = 47 nF).
V. SIMULATION MODELS AND RESULTS FOR FORWARD TYPE DCDC CONVERTER
The simulation model was first verified by comparing the measured waveforms with simulated waveforms, see Figs. 7 and 8. As it can be seen from Figs. 7 and 8, responses to load changes are highly similar. The maximum deviations in load changes are approximately on the same level as well as the corresponding response times.
The goal of these simulations is to outline how high a sampling frequency is needed in a digital voltage controller in order to keep the dynamic performance of the converter on the same level as it is with the currently implemented analog controller. This information is obtained by transforming the analog controller into the z-domain and examining the lowest sampling frequency which gives satisfactory results in terms of dynamics.
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A continuous mode low-frequency model is used to examine the behavior of the converter at low frequencies, i.e., much lower than half of the switching frequency. Fig. 9 (a) presents the forward converter and (b) its low-frequency model. In Fig. 9, NI and N2 are the number of turns in the transformer's primary and secondary windings, respectively.
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Fig. 7. Measured intermediate voltage deviation and input current waveforms.
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Fig. 9. (a) Forward converter and (b) continuos mode behavior model.
Fig. 8. Simulated intermediate voltage deviation and rectified input current waveforms (SIMULINK).
By means of the simulation results, the dynamic requirements for digital controllers were specified in terms of the required sampling rate. The continuous-time transfer function was transformed to the discrete-time domain. With this particular controller scheme, it was found that the sampling frequency of 500 Hz was adequate to give comparable dynamic characteristics as the analog voltage controller. Therefore, our the conclusion is that a reasonably low sampling frequency can be used in PFC's voltage controller.
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Fig. 10 illustrates the control scheme in SIMULINK environment. The current controller is a PI controller which controls the averaged inductor current. Current controller is tuned to be so fast that it controls the current almost ideally (K,= 10, Ti = 0.001). The output signal of the current controller is the duty cycle D, which is allowed to vary in the range [0, O S ] . The input voltage U,, i.e., the intermediate voltage multiplied by the transformer turn ratio, is kept constant. For example, the ripple component at twice the line-frequency does not cause any variation in the output voltage due to the fast current controller. Therefore, the constant input voltage is a justified simplification.
done by lowering the total gain. Lower gain, however, leads to larger, though reasonable, output voltage deviation.
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Fig. 10. Control scheme of the DC/DC converter in SIMULINK environment.
To get a meaningful verification for the simulation model, the same parameters as in the existing 500 W rectifier were used. The SIMULINK model uses the converter parameters presented in Table 11. Verification is presented in Figs. 11 and 12. Load was changed from 10 % to 100 % of the nominal power, and vice versa, TABLE I1 DC/DC CONVERTER PARAMETERS.
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Again, the continuous-time transfer function was transformed into the discrete-time domain. By means of the simple simulation model of the forward converter the goal of this study was achieved. The level of the required sampling frequency was obtained. Without any modifications, the sampling frequency needed to be over 100 kHz which is in practice much too high for a DSP. However, according to the further examinations, it was shown that by means of a modified controller lower sampling frequency was allowed (lower limit is in the range of 20 - 25 kHz). In the modified controller, low-pass filtering was increased in the output of the voltage controller. Another modification was
1998 IEEE
There are still many control applications where the only solution is to implement an analog controller. Analog controllers can be used for very high bandwidth systems. They also give high resolution of a measured signal, and therefore provide precise control. Possible adaptation, for example to a varying operation point, is one of the advantages of the digital controller. Satisfactory performance can then be obtained within a larger operation range. The overall integration level can be increased when implementing a processor-based control system. This results in reduced PCB area required for control circuitry. The same integrated processor-chip can perform several tasks which would require own discrete circuitry in analog implementation. As a more 'intelligent' controller, a digital computer can provide improved diagnostic capabilities, which forms the basis for better maintainability and higher reliability. Also, a faster manufacturing process can be achieved, e.g., by increasing the level of automatic tuning of various parameters. Analog controllers require very often tuning done by potentiometers. This takes more time and also causes an unreliability factor in the production phase.
Fig. 11. Measured output voltage deviation and output current waveforms.
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The advantages of digital control are mainly due to programmability of digital processors as well as their computing and communication capabilities [ 2 ] .First of all, flexibility is the key word related to digital control. The fact that processor-based control systems are programmable gives the designer a possibility to modify the design or its parameters without changing the hardware. Analog controllers are hard-wired solutions making modifications and upgrades in the design difficult, because also the PCB layout has to be modified.
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The simulation results answered to our primary question: how high sampling frequencies are required in the voltage controllers. However, if only the voltage controller is digital, a specific modulator IC is still needed. In addition, a digital to analog converter (DAC) is needed to construct the analog current reference signal which is fed to the modulator circuit. However, fast and high resolution D/A converters are expensive components. It should also be reminded that both analog and digital controllers require their own input and output interface circuitry. A much more compact and also more reliable controller configuration would be achieved if the current controller was implemented digitally as well. Digitally implemented peak current mode control is not a practical alternative due to unreasonably high sampling frequency requirements, but instead the average current mode control should be studied. One idea in digital current control could be that sampling would be more like a 'checking' action. In digital controller, by means of knowing the instantaneous duty cycle and
the moment when the sample is taken, the average value of the current can be estimated. By calculating the current estimates between two successive samples DSP can control the instantaneous duty cycle. Sampling as the checking action is used only to keep the error of the estimates small.
More compact and cost effective solutions for a digital controller in DCDC converters can be constructed if DSP controllers will be equipped with more accurate PWM generators. Also the suitability of application specific integrated circuits (ASIC) as well as the use of new control techniques based on fuzzy logic and neural networks should be studied. In the near future, it will be interesting to see what will be the role of fuzzy control in the control of practical power supplies [12].
However, if we succeed to implement digital current controller, there are still some limitations to be considered. For example, some control and monitoring functions in power supplies require rapid response, and analog comparator circuitry is needed to detect the change of the controlled signal. The logic level change in the output signal of the comparator can then be used to cause an interrupt in the running software rcutine, and the particular ‘emergency’ functions will be performed. The overload-protection is one example which requires fast detection.
It can be stated that digital control will be a very notable alternative in power supplies. However, before digital controllers will become dominant in high-volume power supply products, low cost microcontrollers or signal processors, which are tailored for the specific application, are needed to reduce the existing cost barrier.
Another limitation arises from the duty cycle resolution when the duty cycle is generated digitally as processor’s output signal. If the resolution of DSP’s PWM generator is 50 ns, it leads to duty cycle resolution of 1% with 100 kHz switching frequency. In our rectifier, this means that the minimum controllable voltage step in rectifier’s output is 1.5 V, which is absc’lutelytoo high. Sixteen bit duty cycle resolution gives 2.29 mV, respectively. Naturally, the higher the switching frequency the higher is the required PWM resolution.
REFERENCES G. C. D. Sousa, B. K. Bose, and J. G. Cleland, “Fuzzy logic based on-line efficiency optimization control of an indirect vector-controlled induction motor drive,” IEEE Trans. Industrial Electronics, vol. 42, no. 2, April 1995, pp. 192-198. B. K. Bose, Microcomputer Control of Power Electronics and Drives, IEEE Press, New York, N Y 1996, p. 640. T. Suntio, P. Vallittu, T. Laurinen, and M. Ikonen, “Design of an AC/DC power supply for telecom applications,” in Proceedings of the I997 Finnish Workshop on Power and Industrial Electronics, Espoo, Finland, August 1997, pp. 85-92.
VII. TRENDS IN DSP DEVELOPMENT In the last few years, it has been seen that microcontrollers and DSPs have been mixing up their features. Some microcontrollers were equipped with DSP capabilities to speed up calculation. and to increase resolution. On the other hand, it has been seen lately that some DSP manufactures have built peripheral modules around the DSP cores (e.g., TMS320C240 from Texas Instruments). Such peripheral modules are, e.g., timers, A/D converters, and PWM outputs. This trend of the DSP development can be seen to be continuing due to a whole new range of control applications, which are or will be anxious to use DSPs as a controller. It has been shown that the control of power supplies can be a very demanding DSP application, both in terms of speed and accuracy requirements. VlII. CONCLUSIONS
In the field of elecLronics, one general ongoing trend is the increasing level of system integration. The increased use of processors and microcontrollers has greatly supported this development. The same trend can be seen in the power supply busineswower supplies will be increasingly equipped with I>SPs and microcontrollers. The performance of the currently implemented analog controllers in D C D C converters is often sufficient, and digital controllers may not lead to any improvements in terms of dynamics. However, the rehability and total cost of the digital c o n t r o l l e r can provide the desired result.
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EN 61000-3-2, Electromagnetic compatibility (EMC), Part 3: Limits, Section 2: Limits for harmonic current emissions (equipment input current 5 16 A per phase). R. Redl, P. Tenti, and J. Van Wyk, ”Power electronics’ polluting effect,” IEEE Spectrum, vol. 34, no. 5, pp. 32-39, May 1997.
R. Redl, ”Power-factor correction in single-phase switched-mode power supplies-an overview,” International Journal of Electronics, vol. 77, no. 5, pp. 555-582, November 1994. L. H. Dixon, ”Average current mode control of switching power supplies,” Application Handbook, Application Note U-140, Unitrode, 1997. N. Mohan, T. Undeland, W. Robbins, Power Electronics: Converters, Applications and Design, John Wiley & Sons, New York, NY: 1995, p. 802. L. Dixon, ”High power factor switching preregulator design optimization,” Power Supply Design Seminar Manual, SEM-1100, Unitrode, 1996.
G. Spiazzi, P. Mattavelli, and L. Rossetto, ”Methods to improve dynamic response of power factor preregulators: an overview,” in Proceedings of the 6th European Power Electronics Conference, Sevilla, Spain, September 1995, pp. 3.754-3.759. P. Vallittu, Digital control of power supplies-rjpportunities and construints, Masters thesis, Helsinki University of Technology, Espoo, Finland, 1997, p. 97. P. Mattavelli, L. Rossetto, G. Spiazzi, P. Tenti, ”General-purpose fuzzy controller for DC-DC converters,” IEEE Trans. Power Electronics, vol. 12, no. 1, January 1997, pp. 79-86.
